It is known to use power lines and power line networks to transmit data. An industry standard protocol referred to as CEBus has been established for data transmission between appliances sensors and control devices in a household over the power network in the household. Utility companies use power lines to transmit data to control and monitoring base stations from sensors that monitor power line equipment, such as transformers and switches, and/or power line operating conditions at different locations along a power line.
Transmitters and receivers (hereinafter denoted collectively as “T/R”s) used to transmit and receive data over a low power line network (up to about 250 volts), such as a typical household network, are often electrically connected to power lines in the network via direct conductive contact with the power lines. For high and medium voltage power line networks, such connections are generally undesirable. Connecting a T/R to such power line by direct conductive contact is usually labor intensive and can often be dangerous.
U.S. Pat. No. 5,933,073 to Shuey, the disclosure of which is incorporated herein by reference, describes a communication system that transmits data over power lines in a household power network in which T/Rs are connected via conductive contacts to a power line and a ground line in the network. U.S. Pat. No. 5,485,040 to Sutterlin, the disclosure of which is incorporated herein by reference, describes a communication system in which “power line communication apparatus” is electrically connected via conductive contacts to the “hot” line and neutral line of a power network of a household to transmit data over the network.
Russian Patent SU 554623, the disclosure of which is incorporated herein by reference, describes a system for generating signals on power lines of a first power line network responsive to signals received on power lines of a second power line network. The first and second power line networks are connected by a power transformer and the system routes the signals from the second power line network around the power transformer. Signals on the second power network are inductively sensed. The system is coupled to power lines in the first power line network using conductive contacts.
PCT Publication WO 98/20658, the disclosure of which is incorporated herein by reference, describes a “Non-Invasive Powerline Communication System” in which T/Rs are coupled capacitively or inductively to a power line in a power line system to transmit and sense signals. Signals are transmitted between a first and a second T/R over a same single power line to which both the transmitting and receiving T/R are coupled. To mount a T/R on a power line, the T/R is stressed mechanically to enlarge an opening in the T/R through which the power line is passed into the T/R. The publication states that it is an object of the invention that the T/R be capable of being installed inexpensively and safely without interrupting service to the customer. However, the publication does not describe how this may be accomplished for high voltage power lines (contact with which is not allowed) and in particular for uninsulated high voltage power lines.
Further systems for inductive and capacitive coupling of a T/R to a power line are described, for example, in PCT publication WO 92/16920, the disclosure of which is incorporated herein by reference.
U.S. Pat. No. 6,407,987 to Abraham, the disclosure of which is incorporated herein by reference, describes couplers that have capacitive circuits serially connected with an air-core transformer. One of the described couplers is a differential capacitive coupler.
Not all coupling methods suitable for low voltage wires (e.g., up to about 250 Volts) may be used for medium voltage wires, i.e., above 2000 volts, generally above 6000 volts. For example, wires carrying higher voltages generally require larger capacitors which add higher distortion levels. In addition, the distance between high voltage wires is generally larger than for low voltage wires and therefore requires a relatively long wire length between the T/R and the high voltage wires. Such long wires generally add high distortion levels (e.g., phase skew) to the transmitted signals.
UK patent application GB 2,048,622, the disclosure of which is incorporated herein by reference, describes a device for reducing the amplitude of a signal in a conductor, in order to allow detection of one wire from a group of wires, without signals from the other wires interfering in the detection. The device includes an inductive sensor for sensing signals from the wire and an inductive signal injector to apply a counter signal to the wire.